Presently there is an interest in developing low cost, highly integrated transceivers. Such transceivers may for example be used in consumer electronic devices such as laptop computers, wireless telephones and personal digital assistants for establishing connectivity between such devices so as to permit interoperability. Alternatively, such transceivers may form a part of so called system-on-a chip devices, which may incorporate sensors, signal processing and communication circuits on a single semiconductor die. One use of systems-on-a-chip is in asset tracking transponders that are affixed to capital equipment to be tracked.
The great increases of late in the use of wireless communication has necessitated increased efforts to conserve and reuse portions of the radio spectrum. Furthermore, given the increase in wireless communication, wireless devices must be designed so as to be able to tolerate increased levels of interference.
Direct Sequence Spread Spectrum (DSSS) has proven to be a spectrum efficient signal method that is capable of operating in high interference communication environments. DSSS is for example used in code division multi-Access (CDMA). In conventional DSSS, a binary data stream consisting of ones and zeros is transformed into a bipolar data signal in which ones are represented by a positive signal level (+1) and zeros are represented by a negative signal level (−1). Such a bipolar data signal is characterized by a data rate. The bipolar data signal is mixed with (multiplied by) a higher rate bipolar spreading code signal that includes bit aligned repetitions of a spreading code. The mixed bipolar signal that results from the mixing operation is used to phase shift key modulate a carrier signal.
A variety of phase shift key modulation techniques are known. One technique that is used in conjunction with DSSS is binary phase shift key (BPSK) modulation. In an exemplary implementation of BPSK for every appearance of one in the mixed bipolar signal a carrier signal is transmitted with zero phase shift, and for every appearance of negative one the carrier is transmitted with a phase shift of Pi radians.
Another technique known as quadrature phase shift key (QPSK) modulation offers twice the data rate of BPSK modulation. In an exemplary implementation of QPSK modulation the mixed bipolar signal is parsed into first and second channel bipolar signals. The first channel bipolar signal is used to BPSK modulate a zero phase (i.e. cosine or in-phase component) of a carrier and the second channel bipolar signal is used to BPSK modulate a Pi/2 phase (i.e. sine or quadrature phase) of the carrier signal. The two BPSK modulated carrier signals are summed before being amplified, filtered and transmitted. Because the two phases of the carrier are orthogonal (sine and cosine being orthogonal), the two channels bipolar signals may be transmitted and recovered without interfering with each other.
In QPSK, there are four possible permutations of the signal values of the first and second channel bipolar signals (00, 01, 10, and 11). The four possible permutations lead to four possible phases when the resulting signals are viewed on a complex plane (or phasor diagram) with the result of the first BPSK operation corresponding to the real axis, and the result of the second BPSK operation corresponding to the imaginary axis. One drawback of QPSK is that simultaneous signal level transitions in the first and second channel bipolar signals will result in a Pi radian jump in the phase of the transmitted signal. Such a large abrupt jump in phase can lead to undesirable out of band signal components.
Another phase shift key signaling technique that is a modification of the QPSK method is known as offset quadrature phase shift key (OQPSK). OQPSK addresses the problems caused by large abrupt phase jumps by offsetting the first and second channel bipolar signals so that only one changes signal level at a time, and consequently phase jumps are limited to Pi/2 radians. OQPSK is useful to some extent in reducing out of band signal components.
Aside from the matters of increasing interference tolerance, and reducing the generation of interference, another matter that must be addressed in order to foster the proliferation of wireless connectivity in electronic devices is the cost of modulators. Unfortunately, traditional transmitters that implement phase shift key modulation, use analog mixers to mix baseband signals with one or more phases of a carrier signal in order to phase shift the carrier.